This invention relates to a wrapped film sealing system for electrical equipment.
Many types of conventional electrical equipment contain a dielectric fluid for dissipating the heat that is generated by energized components of the equipment, and for insulating those components from the equipment enclosure and from other internal parts and devices. Examples of such equipment include transformers, capacitors, regulators, circuit breakers and reclosers. Transformers are used extensively in the transmission of electrical power, both at the generating end and at the load end of the power distribution system. A distribution transformer is one that receives electrical power at a first voltage and delivers it at a second, lower voltage.
A distribution transformer consists generally of a core and conductors that are wound about the core so as to form at least two windings. The windings (also referred to as coils) are insulated from each other, and are wound on a common core of magnetically suitable material, such as iron or steel. The primary winding or coil receives energy from an alternating current (AC) source. The secondary winding receives energy by mutual inductance from the primary winding and delivers that energy to a load that is connected to the secondary winding. The core provides a circuit or path for the magnetic lines of force (magnetic flux) which are created by the alternating current flow in the primary winding and which induce the current flow in the secondary winding. The core and winding are typically retained in an enclosure for safety and to protect the core and coil assembly from damage caused by weather or vandalism.
Transformers generate heat during operation through (1) electrical resistance in the conductors that constitute the windings, (2) alternating magnetic flux generating current flow in the core material as the flux passes through the core, and (3) hysteresis (i.e., the friction between the magnetic molecular particles in the core material as they reverse their orientation within the core steel, which occurs when the direction of the AC magnetic field reverses). The generated heat reduces transformer life by degrading the insulation of various internal components, which can lead to an internal fault or short circuit. To dissipate the heat, transformers may be filled with a dielectric coolant, which also functions to electrically insulate the transformer components from one another and from the enclosure.
An electrical connection is formed from the inside of the transformer to the outside using an electrical bushing, such as an insulated component bushing well or tri-clamp bushing. The bushing must provide a seal through an internal stud or components and an external flange. The external flange is sealed by additional gasket components or welding to the flanges.
In one general aspect, a wrapped film sealing system includes a conductive stud, a film layer wrapped around at least a portion of the length of the conductive stud, and a bushing well including a channel passing between two open ends. The conductive stud passes through the channel and a seal is formed between the conductive stud, the film layer, and the bushing.
Embodiments may include one or more of the following features. For example, the conductive stud may include a knurled portion and the film layer may be wrapped around the knurled portion. The knurled portion may include knurled surfaces interspersed with smooth surfaces. The conductive stud may include a smooth portion and the film layer may be wrapped around a portion of the smooth portion. The film layer may include an adhesive layer, such as a heat shrinkable plastic. The film layer also may include a thermoplastic.
The bushing may include a thermoplastic, which may be a nylon. The bushing also may include a thermoset material. The bushing may be a bushing well or a tri-clamp bushing. The conductive stud in the channel in the bushing well may be a removable conductive stud.
In another general aspect, a method of sealing a stud in a bushing includes providing a conductive stud and a film. The film is wrapped around a circumference of the stud along at least a portion of a length of the stud, and the wrapped stud is inserted into a molding machine into which a plastic is then injected. The plastic defines a bushing having a channel through which the stud and film extend. The plastic also bonds to the film such that the film forms a seal in the channel between the stud and bushing.
Embodiments may include one or more of the following features. For example, the method may further include heating the film wrapped around the stud before inserting the stud into the molding machine, such that heating the wrapped film shrinks the wrapped film around the stud. The wrapped film may include an adhesive layer and a heat shrinkable plastic, such as a thermoplastic. The plastic also may include a thermoplastic, which may be nylon, or a thermoset material. The molding machine may be an injection molding machine or a transfer molding machine.
Inserting the stud into the molding machine may include inserting the stud into a mold and placing the mold in the molding machine. The portion of the length of the stud may include a knurled section, with the film being wrapped around the knurled section. The knurled section may include knurled surfaces interspersed with smooth surfaces. The bushing may be a bushing well or a tri-clamp bushing.
The wrapped film sealing system provides considerable advantages. For example, the system may be used to provide a seal between a conductive stud and a bushing to prevent leakage of dielectric fluid. The wrapped film layer can compensate for the difference in thermal expansion between the conductive stud and the plastic bushing, which improves the reliability of the seal.
Conventionally, the seal is provided by spraying an adhesive on the conductive stud and then the bushing is injection molded around the stud. The adhesive may include a solvent that contains potentially environmentally harmful organic solvents that are released into the atmosphere during the spraying step. After the adhesive is applied to the stud, it is baked to cure the adhesive and bond the adhesive to the stud. The wrapped film sealing system advantageously avoids use of potentially harmful solvents, and also avoids the time and expense of baking, thereby resulting in a less complex and much cleaner process.
Other features and advantages will be apparent from the following description, including the drawings, and from the claims.